Topics

Tek 7904 Display Problem


Don Crano <donc@...>
 

Hi All

My old faithful 7904 has started to have a little problem with the display.

It will run for hours or days with no problem. Then it will start to drift
the display up and down, this is the trace/s and readouts as well. Note this
is not on oscillation up and down. It will shift up for awhile, then drop
down lower then it should be for awhile. I am talking about a 1/2 CM or so.
Everything is always on screen, it just gets annoying after awhile.

I found I can turn off the readouts and it will stop. Turn the readouts back
on, and it will be fine again for a few hours or days.

Any ideas what might be going on and where to start looking before I tear
into it.



Thanks,
Don

BTW Hi Mark Gurries glad to see you on the list.


Mark Gurries <gurriesm@...>
 

BTW Hi Mark Gurries glad to see you on the list.
Hi Don.

Been watching the list for a while. It was from one of your email on the
DCC list that I learned about this list. Anyway...

I told Jim Williams here at Linear Tech about this list. (My Lab bench
is next to his.) But he is not one to use PC's. That Jim for ya. For
those who are not familiar with Jim, he is one of out treasured assets at
the company. Hi writes many application notes for our IC and tackling
problems and parts that cause most of us to glaze over our eyes in terms
of the accuracy or noise level requirements. Things like how do you
verify all 24 bits of a 24 bit DAC!

The trick around here to getting a Tek scope fixed and checked out by Jim
Williams is to get an old 500 or 400 series scope in the lab. Tell Jim
about it, and the fact that it does not work annoys Jim to no end.
Before you know it, he is digging into the scope and fixing everything.
Not that we cannot fix out own scopes mind you, but, Jim does a faster
and better job than anyone I know. It fun to watch him and of course you
learn along the way the history and debug tricks.

We have about every make of Tek scope at the company labs. We have lots
of 547's working in parallel with the latest Tek Scopes. We just got
around to setting up an old 556 dual beam on another engineers lab bench.
454's are another favorite. A few of the guys have 7K stuff like yours.
One of my employees has every 7K plug in made including some test
adapters if I recall correctly. It amazing to see the latest scopes
setup right next to an old classic. The biggest reason we like the old
400 and 500 series scopes is the very fine trace and excellent noise
floor. When Jim sets up his 556 with two 1A4 plugins showing 8 traces
that are all beautifully detailed (4 of the traces are on a different
time base to boot) published in a App note, we sometimes get letters and
calls about the scope photo alone! How did you do that?

We think that our lab is one of the biggest consumers of electricity in
Silicon Valley with all these Tube Tek scopes on and possibly the source
of all the blackouts here in California. We already went though one air
conditioner! ;-0


Best Regards,

Mark Gurries
gurriesm@macconnect.com

---------------------------------------------------------
Promotor of the DCC Standard for Model trains.
Model Railroad Club and NMRA DCC presentations are at:
http://www.falken.net/dave/svl
--------------------------------------------------------
Audio Enthusiast (Love SAE equipment)
http://members.ebay.com/aboutme/gurriesm/
http://www.wardsweb.org/audio/index_audio.html
----------------------------------------------------------


Don Crano <donc@...>
 

Hi Mark,

Yeah I remember posting a list of the YaHoo groups I belong to.

I know what you mean about the older scopes and their fine trace lines. My
old 453 is still on of my favorites, I can sync it on anything I feed into
it.<g> Under it sits a 475A with it's mixed mode delay is nice, but does
not have the fine trace of the ol 453. On a Tekmobile 3 I have the 7904,
with the two lower shelfs each holding a TM504A, and both fully loaded with
plug-ins. So it is my mobile work bench, I can take it any where around the
layout as need be.<g> I really do not have a count of the 7K and TM plug-ins
I have and what works and does not work. But I am sure I do not have them
all. Then on my bench I have a 2230 for what ever analog/digital I might
need.

Don

Remember Always Have Fun and Enjoy!,
Don Crano Akron, Oh NMRA #096211
mailto:donc@sssnet.com
Visit Model Railroading with DCC at:
http://pages.sssnet.com/donc/
Join Model Railroading with DCC Web Ring.

----- Original Message -----
From: Mark Gurries <gurriesm@macconnect.com>
To: <TekScopes@yahoogroups.com>
Sent: Thursday, April 19, 2001 6:12 PM
Subject: Re: [TekScopes] Tek 7904 Display Problem


BTW Hi Mark Gurries glad to see you on the list.
Hi Don.

Been watching the list for a while. It was from one of your email on the
DCC list that I learned about this list. Anyway...

I told Jim Williams here at Linear Tech about this list. (My Lab bench
is next to his.) But he is not one to use PC's. That Jim for ya. For
those who are not familiar with Jim, he is one of out treasured assets at
the company. Hi writes many application notes for our IC and tackling
problems and parts that cause most of us to glaze over our eyes in terms
of the accuracy or noise level requirements. Things like how do you
verify all 24 bits of a 24 bit DAC!

The trick around here to getting a Tek scope fixed and checked out by Jim
Williams is to get an old 500 or 400 series scope in the lab. Tell Jim
about it, and the fact that it does not work annoys Jim to no end.
Before you know it, he is digging into the scope and fixing everything.
Not that we cannot fix out own scopes mind you, but, Jim does a faster
and better job than anyone I know. It fun to watch him and of course you
learn along the way the history and debug tricks.

We have about every make of Tek scope at the company labs. We have lots
of 547's working in parallel with the latest Tek Scopes. We just got
around to setting up an old 556 dual beam on another engineers lab bench.
454's are another favorite. A few of the guys have 7K stuff like yours.
One of my employees has every 7K plug in made including some test
adapters if I recall correctly. It amazing to see the latest scopes
setup right next to an old classic. The biggest reason we like the old
400 and 500 series scopes is the very fine trace and excellent noise
floor. When Jim sets up his 556 with two 1A4 plugins showing 8 traces
that are all beautifully detailed (4 of the traces are on a different
time base to boot) published in a App note, we sometimes get letters and
calls about the scope photo alone! How did you do that?

We think that our lab is one of the biggest consumers of electricity in
Silicon Valley with all these Tube Tek scopes on and possibly the source
of all the blackouts here in California. We already went though one air
conditioner! ;-0


Best Regards,

Mark Gurries
gurriesm@macconnect.com

---------------------------------------------------------
Promotor of the DCC Standard for Model trains.
Model Railroad Club and NMRA DCC presentations are at:
http://www.falken.net/dave/svl
--------------------------------------------------------
Audio Enthusiast (Love SAE equipment)
http://members.ebay.com/aboutme/gurriesm/
http://www.wardsweb.org/audio/index_audio.html
----------------------------------------------------------


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patrick_vandenberghe@...
 

--- In TekScopes@y..., "Don Crano" <donc@s...> wrote:
Hi All

My old faithful 7904 has started to have a little problem with the
display.

It will run for hours or days with no problem. Then it will start
to drift
the display up and down, this is the trace/s and readouts as well.
Note this
is not on oscillation up and down. It will shift up for awhile,
then drop
down lower then it should be for awhile. I am talking about a 1/2
CM or so.
Everything is always on screen, it just gets annoying after awhile.

I found I can turn off the readouts and it will stop. Turn the
readouts back
on, and it will be fine again for a few hours or days.
Hello Don,

I had this problem. It turned out to be the channel switch hybrid
U625 (part nr. 1555-0060). This is located on a 'mezzanine' PCB
behind the two vertical plugins.
Some other sources mentioned the two 1W metal film resistors on the
vertical amplifier.

hope this helps

Patrick